The present invention relates generally to fuel combustion apparatus and, in preferred embodiments thereof, more particularly relates to apparatus and methods for reducing the NOx emissions of fossil fuel combustion systems such as those incorporated in water heaters and forced air heating furnaces.
In order to assure a more complete, fuel-efficient combustion of the air/fuel mixture supplied to the burner system of a fuel-fired water heater, forced air heating furnace or the like, it is common practice in the heating appliance industry to adjust the air-to-fuel ratio of the mixture to a higher than stoichiometric ratio--i.e., an "excess air" condition. Due to the nitrogen content of combustion air used as the oxygen source, an undesirable result frequently associated with the overall combustion process is the ultimate harmful emission to atmosphere of nitrogen oxides (NOx).
As is well known, the amount of emitted NOx increases as combustion temperatures increase, and also increases with higher air-to-fuel ratios of the pre-combustion air/fuel mixture. Accordingly, the conventional approach of supplying an air-rich air/fuel mixture to the heating appliance's combustion system produces both a hot combustion flame and excess air, thus compounding the NOx generation problem.
A conventional technique for reducing the amount of NOx created in the flame of a fossil fuel heating appliance is that of "quenching" the flame--i.e., inserting a heat absorbing member into the flame to conduct heat away from portions thereof. This reduces the maximum temperature attained and, consequently, the amount of NOx created in the combustion process. However, the excess or near-stoichiometric air quantity initially present in the pre-combustion air/fuel mixture still tends to undesirably elevate the level of NOx generated.
In view of the foregoing, it is accordingly an object of the present invention to provide an improved combustion system for fuel-fired heating appliances that achieves lowered NOx emission rates without the use of flame quenching techniques.